1. Field of the Invention
The present invention relates to a method of producing a single-polarized lithium tantalate crystal and a single-polarized lithium tantalate crystal which is used for the application wherein a pattern is formed with a metal electrode on a wafer to process an electrical signal such as a surface acoustic wave device.
2. Description of the Related Art
Lithium tantalate is used for the application which utilizes its electrical properties, such as a SAW device which performs a signal processing using a surface acoustic wave (SAW). In this application, the single-polarized lithium-tantalate crystal is used. The lithium-tantalate crystal suitable for this purpose shows a piezoelectric response (piezoelectric property) originated from the crystal structure thereof and needed for a SAW device. However, the lithium-tantalate crystal obtained by a general method has a pyroelectric response (pyroelectric property), in addition to the piezoelectric property.
A piezoelectric property of a lithium-tantalate crystal is essential characteristic when using the lithium-tantalate crystal as a SAW device. On the other hand, a pyroelectric property is observed as a surface charge generated on an external surface of a lithium-tantalate crystal by applying a temperature change to the crystal, and charges the crystal with electricity. It is considered that, when using a lithium-tantalate crystal as a SAW device, the surface charge causes a spark discharge between metal electrodes formed on the wafer consisting of a lithium-tantalate crystal, which causes a significant defect in a performance of the SAW device. For this reason, in a design of a SAW device using a lithium-tantalate crystal, there are needed an artifice for preventing the generation of a surface charge, an artifice for discharging the surface charge, an artifice for making an interval between metal electrodes large, or the like. There is the disadvantage that the design of the SAW device itself is restricted in order to take these artifices in the design.
Moreover, in the process of producing a SAW device using a lithium-tantalate crystal, there is a process of heating the lithium-tantalate crystal in processes such as deposition of a metal film and removal of a resist. If a temperature change such as elevation or lowering of temperature is applied to the lithium-tantalate crystal in these processes, a charge will be generated on the external surface due to the pyroelectric property of the lithium-tantalate crystal. As mentioned above, a spark discharge is generated between metal electrodes due to this surface charge, and causes the breakage of the electrode pattern. Therefore, artifices are considered so that a temperature change may not be applied as possible, or so that a temperature change may be gentle in the process of producing the SAW device. These artifices may cause disadvantages that the throughput of the production process is lowered, and that the margin which guarantees the performance of the SAW device becomes narrow.
Although the charge on the external surface generated due to the pyroelectric property is neutralized by the free charge from the surrounding environment and decayed with time in the single-polarized lithium-tantalate crystal produced by the usual method, the decay time is long as several hours or more, and thus it is not industrial to decay the generated surface charge by such natural neutralization in the process of producing the SAW device.
From the above-mentioned background, there has been increased a demand for the piezoelectric crystal in which the generation or accumulation of the charge is not observed on the external surface of the crystal with maintaining the piezoelectric property needed, in order to achieve device characteristics for the application like a SAW device, and there has been needed a single-polarized lithium-tantalate crystal in which the accumulation of the surface charge is not observed for such a application.
As a method for producing a lithium-tantalate crystal in which the surface charge is decreased, there is disclosed a method wherein a lithium-tantalate crystal is exposed to a reducing atmosphere at 500° C. or more to increase the electric conductivity, and thereby the surface charge generated due to the pyroelectric property is quickly neutralized or decayed (see Japanese Patent Application Laid-open (Kokai) No. 11-92147 (Table 1)).